What Is Cement? Manufacturing, Constituents & their Functions
Material with adhesive and cohesive properties. Any material that binds or unites – essentially like glue
Function Of Cement
- to bind the sand and coarse aggregate together
- to fill voids in between sand and coarse aggregate particle
- to form a compact mass
Types of Cement
2 types of cement normally used in building industry are as follows:
- a) Hydraulic Cement
- b) Nonhydraulic Cement
Hydraulic Cement sets and hardens by action of water. Such as Portland Cement
In other words it means that hydraulic cement are:
“ Any cements that turns into a solid product in the presence of water (as well as air) resulting in a material that does not disintegrate in water.”
Any cement that does not require water to transform it into a solid product.
2 common Non-hydraulic Cement are
- a) Lime – derived from limestone / chalk
Portland Cement – constituents of cement
Chemical composition of Portland Cement:
- a) Tricalcium Silicate (50%)
- b) Dicalcium Silicate (25%)
- c) Tricalcium Aluminate (10%)
- d) Tetracalcium Aluminoferrite (10%)
- e) Gypsum (5%)
Functions of Tricalcium Silicate
- Hardens rapidly and largely responsible for initial set & early strength
- The increase in percentage of this compound will cause the early strength of Portland Cement to be higher.
- A higher percentage of this compound will produces higher heat of hydration and accounts for faster gain in strength.
Functions of Dicalcium Silicate
- Hardens slowly
- It effects on strength increases occurs at ages beyond one week .
- Responsible for long term strength
Functions of Tricalcium Aluminate
- Contributes to strength development in the first few days because it is the first compound to hydrate .
- It turns out higher heat of hydration and contributes to faster gain in strength.
- But it results in poor sulfate resitance and increases the volumetric shrinkage upon drying.
- Cements with low Tricalcium Aluminate contents usually generate less heat, develop higher strengths and show greater resistance to sulfate attacks.
- It has high heat generation and reactive with soils and water containing moderate to high sulfate concentrations so it’s least desirable.
Function of Tetracalcium Aluminoferrite
- Assist in the manufacture of Portland Cement by allowing lower clinkering temperature.
- Also act as a filler contributes very little strength of concrete eventhough it hydrates very rapidly.
- Also responsible for grey colour of Ordinary Portland Cement
Manufacturing Of Portland Cement
The 3 primary constituents of the raw materials used in the manufacture of Portland Cement are:
- a) Lime
Lime is derived from limestone or chalk
Silica & Alumina from clay, shale or bauxite
There are 2 chief aspects of the manufacturing process:
To produce a finely divided mixture of raw materials – chalk / limestone and clay / shale
To heat this mixture to produce chemical composition
There 2 main process that can be used in manufacturing of Portland Cement that is
- i) wet process ii) dry process
Raw materials are homogenized by crushing, grinding and blending so that approximately 80% of the raw material pass a No.200 sieve. The mix will be turned into form of slurry by adding 30 – 40% of water. It is then heated to about 2750ºF (1510ºC) in horizontal revolving kilns (76-153m length and 3.6-4.8m in diameter. Natural gas, petroluem or coal are used for burning. High fuel requirement may make it uneconomical compared to dry process.
Raw materials are homogenized by crushing, grinding and blending so that approximately 80% of the raw material pass a No.200 sieve. Mixture is fed into kiln & burned in a dry state. This process provides considerable savings in fuel consumption and water usage but the process is dustier compared to wet process that is more efficient than grinding.
DRY PROCESS & WET PROCESS
In the kiln, water from the raw material is driven off and limestone is decomposed into lime and Carbon Dioxide.
limestone = lime + Carbon Dioxide
In the burning zone, portion of the kiln, silica and alumina from the clay undergo a solid state chemical reaction with lime to produce calcium aluminate.
silica & alumina + lime = calcium aluminate
The rotation and shape of kiln allow the blend to flow down the kiln, submitting it to gradually increasing temperature. As the material moves through hotter regions in the kiln, calcium silicates are formed. These products, that are black or greenish black in color are in the form of small pellets, called cement clinkers. Cement clinkers are hard, irregular and ball shaped particles about 18mm in diameter. The cement clinkers are cooled to about 150ºF (51ºC) and stored in clinker silos.
When needed, clinker are mixed with 2-5% gypsum to retard the setting time of cement when it is mixed with water. Then, it is grounded to a fine powder and then the cement is stored in storage bins or cement silos or bagged. Cement bags should be stored on pallets in a dry place.